Method for producing honeycomb structure and method for producing electrically heating support

ABSTRACT

A method for producing a honeycomb structure includes: a forming step of extruding a forming raw material containing a ceramic raw material to obtain a honeycomb formed body, the honeycomb formed body including: an outer peripheral wall; and partition walls disposed on an inner side of the outer peripheral wall, the partition walls defining a plurality of cells, each of the plurality of cells extending from one end face to the other end face to form a flow passage; a drying step of drying the honeycomb formed body to obtain a honeycomb dried body; and a firing step of firing the honeycomb dried body to obtain a honeycomb fired body. The forming step includes extruding the forming raw material to produce a honeycomb formed body in which a part of the partition walls is lost so that some of the cells are connected to each other.

FIELD OF THE INVENTION

The present invention relates to a method for producing a honeycombstructure, and a method for producing an electrically heating support.

BACKGROUND OF THE INVENTION

Recently, electrically heated catalysts (EHCs) have been proposed toimprove a decrease in exhaust gas purification performance immediatelyafter engine starting. For example, the EHC is configured to connectmetal electrodes to a pillar shaped honeycomb structure made ofconductive ceramics, and conducting a current to heat the honeycombstructure itself, thereby enabling a temperature to be increased to anactivation temperature of the catalyst prior to the engine starting.

Since the EHCs are subjected to heat and/or impact from an engine, theyare required to have good thermal shock resistance. If cracks aregenerated in the honeycomb structure of the EHC due to heat and/orimpact from the engine, the energization passage in the honeycombstructure is changed and localized heat is generated, resulting indegradation of the catalyst. Further, the energization resistanceincreases, which will be difficult to control the current flow. As aresult, an exhaust gas purification efficiency of the EHC may bedeteriorated.

Patent Literature 1 discloses a honeycomb structure having improvedthermal shock resistance by forming slits that open on a side surface ofa honeycomb structure portion. In Patent Literature 1, a honeycomb driedbody is formed, and the slits are then formed by cutting partition wallsof the honeycomb dried body with Leutor or the like.

Patent Literature 2 discloses a method for forming a slit(s) on an endface of a honeycomb structure. Specifically, it forms the slit byarranging a slit-forming plate member so as to be in contact with oneend face of a honeycomb formed body, and moving the slit-forming platemember toward the other end face of the honeycomb formed body whilevibrating the slit-forming plate member to cut partition walls of thehoneycomb formed body.

CITATION LIST Patent Literatures

-   [Patent Literature 1] Japanese Patent No. 5997259 B-   [Patent Literature 2] Japanese Patent No. 5162509 B

SUMMARY OF THE INVENTION

Both of the techniques disclosed in Patent Literatures 1 and 2 requirethe step of forming the slit in the method for producing the honeycombstructure, and the number of operation steps increases accordingly, sothat a production efficiency decreases. Further, they have a problem ofwear and damage of processing tools for slit formation or the like,which may increase the production cost.

The present invention has been made in light of the above circumstances.A problem of the present invention is to provide a method for producinga honeycomb structure and an electrically heating support, which canform at least one slit in a honeycomb structure with a good productionefficiency and production cost.

The above problem is solved by the following present disclosure, and thepresent disclosure is specified as follows:

(1) A method for producing a honeycomb structure, the method comprising:

a forming step of extruding a forming raw material containing a ceramicraw material to obtain a honeycomb formed body, the honeycomb formedbody comprising: an outer peripheral wall; and partition walls disposedon an inner side of the outer peripheral wall, the partition wallsdefining a plurality of cells, each of the plurality of cells extendingfrom one end face to the other end face to form a flow passage;

a drying step of drying the honeycomb formed body to obtain a honeycombdried body; and

a firing step of firing the honeycomb dried body to obtain a honeycombfired body,

wherein the forming step comprises extruding the forming raw material toproduce a honeycomb formed body in which a part of the partition wallsis lost so that some of the cells are connected to each other.

(2) A method for producing a honeycomb structure, the method comprising:

a forming step of extruding a forming raw material containing a ceramicraw material to obtain a honeycomb formed body, the honeycomb formedbody comprising: an outer peripheral wall; and partition walls disposedon an inner side of the outer peripheral wall, the partition wallsdefining a plurality of cells, each of the plurality of cells extendingfrom one end face to the other end face to form a flow passage;

a drying step of drying the honeycomb formed body to obtain a honeycombdried body; and

a firing step of firing the honeycomb dried body to obtain a honeycombfired body,

wherein the forming step comprises extruding the forming raw material toform a honeycomb formed body in which a part of the partition walls isformed thinner than the other partition walls and arranged in a form ofa slit.

(3) The method for producing the honeycomb structure according to (1) or(2), wherein the method further comprises the steps of:

applying an electrode portion forming raw material containing a ceramicraw material to a side surface of the honeycomb dried body, and dryingthe applied electrode portion forming raw material to obtain a honeycombdried body with unfired electrode portions; and

firing the honeycomb dried body with unfired electrode portions toobtain a honeycomb structure having a pair of electrode portions, and

wherein the pair of the electrode potions are arranged on an outersurface of the outer peripheral wall across a central axis of thehoneycomb dried body so as to extend in a form of strip in a flowpassage direction of the cells.

(4) A method for producing an electrically heating support, wherein themethod comprises a step of electrically connecting a metal electrode toeach of the pair of electrode portions of the honeycomb structureproduced by the method according to (3).

According to the present invention, it is possible to provide a methodfor producing a honeycomb structure and an electrically heating support,which can form at least one slit in a honeycomb structure with a goodproduction efficiency and production cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic external view of a honeycomb structure accordingto an embodiment of the present invention;

FIG. 2 is a schematic cross-sectional view of an electrically heatingsupport according to an embodiment of the present invention, which isperpendicular to an extending direction of cells;

FIG. 3 is specific examples of slit shapes of honeycomb structuresaccording to an embodiment of the present invention;

FIG. 4 (A) is a top view [1], a side view [2], and a bottom view [3] ofa U-shaped pin; and FIG. 4 (B) is a top view [1], a side view [2], and abottom view [3] of a T-shaped pin;

FIG. 5 (A) is a schematic plane view for explaining a state where a slitof a honeycomb formed body is formed using a U-shaped pin; and FIG. 5(B) is a schematic cross-sectional view of the U-shaped pin and a die inthe state corresponding to FIG. 5 (A);

FIG. 6 (A) is a schematic plane view for explaining a state where a slitof a honeycomb formed body is formed by using a T-shaped pin; and FIG. 6(B) is a schematic cross-sectional view of the T-shaped pin and a die inthe state corresponding to FIG. 6 (A);

FIG. 7 (A) is a schematic plane view of a honeycomb formed body whichhas cells each having a quadrangular cross section and which has a slitformed; and FIG. 7 (B) is a schematic plane view of a honeycomb formedbody which has cells each having a hexagonal cross section and which hasa slit formed;

FIG. 8 is a schematic plane view of a die having a closed portion;

FIG. 9 is a schematic plane view of a die having a hole formed smallerthan other holes;

FIG. 10 is a schematic cross-sectional view of a molding machine forexplaining a step of forming a kneaded material in a molding machine;

FIG. 11 (A) is a schematic plane view of a die used in Example 1; FIG.11 (B) is a schematic plane view of slits produced by FIG. 11 (A); FIG.11 (C) is a schematic plane view of a die used in Example 2; and FIG. 11(D) is a schematic plane view of a slit produced by FIG. 11 (C);

FIG. 12 (A) is a schematic plane view of a die used in Example 3; andFIG. 12 (B) is a schematic plane view of slits produced by FIG. 12 (A);and

FIG. 13 (A) is a schematic plane view of a honeycomb formed body whichhas cells each having a quadrangular cross section and which has a slitformed; and FIG. 13 (B) is a schematic plane view of a honeycomb formedbody which has cells each having a hexagonal cross section and which hasa slit formed.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments according to the present invention will bespecifically described with reference to the drawings. It is tounderstand that the present invention is not limited to the followingembodiments, and various design modifications and improvements may bemade based on ordinary knowledge of a person skilled in the art, withoutdeparting from the spirit of the present invention.

(1. Honeycomb Structure)

FIG. 1 is a schematic external view of a honeycomb structure 10according to an embodiment of the present invention. The honeycombstructure 10 includes a pillar shaped honeycomb structure portion 11 andelectrode portions 13 a, 13 b. The honeycomb structure 10 may notinclude the electrode portions 13 a, 13 b.

(1-1. Pillar Shaped Honeycomb Structure Portion)

The pillar shaped honeycomb structure partition 11 includes: an outerperipheral wall 12; and partition walls 19 which are disposed on aninner side of the outer peripheral wall 12 and define a plurality ofcells 18 each extending from one end face to the other end face to forma flow passage.

An outer shape of the pillar shaped honeycomb structure portion 11 isnot particularly limited as long as it is pillar shaped. For example,the honeycomb structure portion can have a shape such as a pillar shapewith circular end faces (cylindrical shape), a pillar shape with ovalend faces and a pillar shape with polygonal (quadrangular, pentagonal,hexagonal, heptagonal, octagonal, etc.) end faces. The size of thepillar shaped honeycomb structure portion 11 is such that an area of theend faces is preferably from 2000 to 20000 mm², and more preferably from5000 to 15000 mm², for the purpose of improving heat resistance(suppressing cracks entering the outer peripheral wall in acircumferential direction).

The pillar shaped honeycomb structure portion 11 is made of a materialselected from the group consisting of oxide ceramics such as alumina,mullite, zirconia and cordierite, and non-oxide ceramics such as siliconcarbide, silicon nitride and aluminum nitride, although not limitedthereto. Silicon carbide-metal silicon composite materials and siliconcarbide-graphite composite materials may also be used. Among them, thematerial of the pillar shaped honeycomb structure portion preferablycontains ceramics mainly based on the silicon-silicon carbide compositematerial or on silicon carbide, in terms of achieving both heatresistant and electrical conductivity. The phrase “the pillar shapedhoneycomb structure portion 11 is mainly based on a silicone-siliconcarbide composite material” as used herein means that the pillar shapedhoneycomb structure portion 11 contains 90% by mass or more of thesilicon-silicon carbide composite material (total mass) based on theentire honeycomb structure portion. Here, the silicon-silicon carbidecomposite material contains silicon carbide particles as an aggregateand silicon as a bonding material for bonding the silicon carbideparticles, and a plurality of silicon carbide particles are preferablybonded by silicon so as to form pores between the silicon carbideparticles. The phrase “the pillar shaped honeycomb structure portion 11is mainly based on silicon carbide” as used herein means that the pillarshaped honeycomb structure portion 11 contains 90% by mass or more ofthe silicon carbide (total mass) based on the entire honeycomb structureportion.

When the pilar shaped honeycomb structure portion 11 contains thesilicon-silicon carbide composite material, a ratio of a “mass ofsilicon as a bonding material” contained in the pillar shaped honeycombstructure portion 11 to the total of a “mass of silicon carbideparticles as an aggregate” contained in the pillar shaped honeycombstructure portion 11 and a “mass of silicon as a bonding material”contained in the pillar shaped honeycomb structure portion 11 ispreferably from 10 to 40% by mass, and more preferably from 15 to 35% bymass.

A shape of each cell in a cross section perpendicular to an extendingdirection of the cells 18 is not limited, but it is preferably aquadrangle, a hexagon, an octagon, or a combination thereof. Amongthese, the quadrangle and the hexagon are preferred, in terms of easilyachieving both structural strength and heating uniformity.

Each of the partition walls 19 defining the cells 18 preferably has athickness of from 0.1 to 0.3 mm, and more preferably from 0.15 to 0.25mm. As used herein. the thickness of the partition wall 19 is defined asa length of a portion passing through the partition walls 19, among linesegments connecting centers of gravity of the adjacent cells 18 in thecross section perpendicular to the extending direction of the cells 18.

The pillar shaped honeycomb structure portion 11 preferably has a celldensity of from 40 to 150 cells/cm², and more preferably from 70 to 100cells/cm², in the cross section perpendicular to the flow passagedirection of the cells 18. The cell density in such a range can increasethe purification performance of the catalyst while reducing the pressureloss upon flowing of an exhaust gas. The cell density is a valueobtained by dividing the number of cells by an area of one end face ofthe pillar shaped honeycomb structure portion 11 excluding the outerperipheral wall 12 portion.

The provision of the outer peripheral wall 12 of the pillar shapedhoneycomb structure portion 11 is useful in terms of ensuring thestructural strength of the pillar shaped honeycomb structure portion 11and preventing a fluid flowing through the cells 18 from leaking fromthe outer peripheral surface of the pillar shaped honeycomb structureportion 11. More particularly, the thickness of the outer peripheralwall 12 is preferably 0.05 mm or more, and more preferably 0.1 mm ormore, and even more preferably 0.15 mm or more. However, if the outerperipheral wall 12 is too thick, the strength becomes too high, so thata strength balance between the outer peripheral wall 12 and thepartition wall 19 is lost to reduce thermal shock resistance, and if thethickness of the outer peripheral wall 12 is excessively increased, theheat capacity increases and a temperature difference between the outerperipheral side and the inner peripheral side of the outer peripheralwall 12 increases, so that the heat impact resistance decreases.Therefore, the thickness of the outer peripheral wall 12 is preferably1.0 mm or less, and more preferably 0.7 mm or less, and still morepreferably 0.5 mm or less. As used herein, the thickness of the outerperipheral wall 12 is defined as a thickness of the outer peripheralwall 12 in a direction of a normal line to a tangential line at ameasurement point when observing a portion of the outer peripheral wall12 to be subjected to thickness measurement in the cross sectionperpendicular to the extending direction of the cells.

The partition walls 19 of the pillar shaped honeycomb structure portion11 preferably have an average pore diameter of from 2 to 15 μm, and morepreferably from 4 to 8 μm. The average pore diameter is a value measuredby a mercury porosimeter.

The partition walls 19 may be porous. When the partition walls 19 areporous, the partition wall 19 preferably has a porosity of from 35 to60%, and more preferably from 35 to 45%. The porosity is a valuemeasured by a mercury porosimeter.

(1-2. Electrode Portion)

The honeycomb structure 10 according to an embodiment of the presentinvention includes a pair of electrode portions 13 a, 13 b on an outersurface of the outer peripheral wall 12 across a central axis of thepillar shaped honeycomb structure portion 11 so as to extend in a formof strip in the flow passage direction of the cells 18. By thusproviding the pair of electrode portion 13 a, 13 b, uniform heatgeneration of the honeycomb structure 10 can be enhanced. It isdesirable that each of the electrode portions 13 a, 13 b extends over alength of 80% or more, and preferably 90% or more, and more preferablythe full length, between both end faces of the honeycomb structure 10,from the viewpoint that a current easily spreads in an axial directionof each of the electrode portions 13 a, 13 b. It should be noted thatthe honeycomb structure may not include the electrode portions 13 a, 13b.

Each of the electrode portions 13 a, 13 b preferably has a thickness offrom 0.01 to 5 mm, and more preferably from 0.01 to 3 mm. Such a rangecan allow uniform heat generation to be enhanced. The thickness of eachof the electrode portions 13 a, 13 b is defined as a thickness in adirection of a normal line to a tangential line at a measurement pointon an outer surface of each of the electrode portions 13 a, 13 b whenobserving the point of each electrode portion to be subjected tothickness measurement in the cross section perpendicular to theextending direction of the cells.

The electric resistivity of each of the electrode portions 13 a, 13 b islower than the electric resistivity of the pillar shaped honeycombstructure portion 11, whereby the electricity tends to flowpreferentially to the electrode portions 13 a. 13 b, and the electricitytends to spread in the flow passage direction and the circumferentialdirection of the cells 18 during electric conduction. The electricresistivity of the electrode portions 13 a, 13 b is preferably 1/10 orless, and more preferably 1/20 or less, and even more preferably 1/30 orless, of the electric resistivity of the pillar shaped honeycombstructure portion 11. However, if the difference in electric resistivitybetween both becomes too large, the current is concentrated between endsof the opposing electrode portions to bias the heat generated in thepillar shaped honeycomb structure portion 11. Therefore, the electricresistivity of the electrode portions 13 a, 13 b is preferably 1/200 ormore, and more preferably 1/150 or more, and even more preferably 1/100or more, of the electric resistivity of the pillar shaped honeycombstructure portion 11. As used herein, the electric resistivity of theelectrode portions 13 a, 13 b is a value measured at 25° C. by afour-terminal method.

Each of the electrode portions 13 a, 13 b may be made of conductiveceramics, a metal, and a composite of a metal and conductive ceramics(cermet). Examples of the metal include a single metal of Cr, Fe, Co,Ni, Si or Ti, or an alloy containing at least one metal selected fromthe group consisting of those metals. Non-limiting examples of theconductive ceramics include silicon carbide (SiC), metal compounds suchas metal silicides such as tantalum silicide (TaSi₂) and chromiumsilicide (CrSi₂). Specific examples of the composite of the metal andthe conductive ceramics (cermet) include a composite of metal siliconand silicon carbide, a composite of metal silicide such as tantalumsilicide and chromium silicide, metal silicon and silicon carbide, andfurther a composite obtained by adding to one or more metals listedabove one or more insulating ceramics such as alumina, mullite,zirconia, cordierite, silicon nitride, and aluminum nitride, in terms ofdecreased thermal expansion.

(1-3. Slit)

In a cross section perpendicular to a flow passage direction of thecells 18 of the honeycomb structure 10, at least one linear slit 21 isprovided. By having such a linear slit 21, cracking on the end faces ofthe honeycomb structure 10 can be suppressed. The provision of the abovelinear slit 21 can relax stress to reduce a thermal expansiondifference, so that the cracking can be well suppressed.

In FIG. 1, the slit 21 indicates a position thereof in the honeycombstructure 10, and its shape is not particularly limited as long as it iselongated. Further, the slit 21 has a shape such that adjacent cells areconnected to each other by removing the partition walls 19 between them.The slit 21 is preferably in a form where the slit extends in theextending direction of the cells and is provided on both end faces.

The shape and number of slits 21 are not particularly limited and can bedesigned as needed. For example, two, or four or more slits 21 may beindependently formed. By having a plurality of slits formedindependently, the generation of cracks in the honeycomb structure 10can be well controlled. The width of each slit 21 are not particularlylimited. The width of the slit may be formed to be the same as the widthof each cell 18, or the width of each slit may be formed smaller orlarger than that of each cell 18. The width of each slit is notparticularly limited, but it may be from 1 to 30 cm. The width of eachslit 21 can be adjusted depending on the size, materials, andapplications of the honeycomb structure 10, and the number of slits andthe length of the slit.

In an embodiment of the present invention, the slit 21 preferably passesthrough the center of the pillar shaped honeycomb structure portion 11in the cross section perpendicular to the flow passage direction of thecells of the pillar shaped honeycomb structure portion 11. Such aconfiguration can lead to better control of changes in resistance andcurrent passage of the honeycomb structure 10. Each slit may be dividedinto sections along an extending direction of the slits. In this case,the slit 21 may be divided into slits having the same length ordifferent lengths. By dividing and forming the slit, the generation ofcracks in the honeycomb structure 10 can be well controlled. The numberof the slits divided is not particularly limited, but each slit 21 maybe divided into two, three, or four or more sections. In addition, thehoneycomb structure may be provided with a plurality of slits consistingof the combination of divided slits and non-divided slits.

A ratio of the length of the slit 21 to an outer diameter of the pillarshaped honeycomb structure portion 11 is preferably 25% or more. Theratio of the length of the slit 21 to the outer diameter of the pillarshaped honeycomb structure 11 of 25% or more can allow thermal shock tobe better relaxed and cracking to be better suppressed.

It is preferable that a depth of the slit 21 in the flow passagedirection of the cells 18 from one end face in the honeycomb structure10 is from 30 to 100% of the full length of the pillar shaped honeycombstructure portion 11. The depth of the slit 21 of from 30 to 100% of thefull length of the pillar shaped honeycomb structure portion 11 can leadto improvement of thermal shock resistance. The depth of the slit 21 ispreferably from 50 to 100%, even more preferably from 70 to 100%, of thefull length of the pillar shaped honeycomb structure portion 11.

Specific examples of shapes of the slits 21 are shown in (A) to (L) ofFIG. 3. It should be noted that each of (A) to (L) of FIG. 3 only showsthe outer diameter of the end face of the pillar shaped honeycombstructure portion 11 and the shape of the slit.

The slit 21 may pass through the center and extend to the outerperiphery on both sides on the end face of the pillar shaped honeycombstructure 11, as shown in FIG. 3 (A), or it may pass through the centerand extend to the middle without reaching the outer periphery, as shownin FIG. 3 (B), or it may pass through the center and have any slope, asshown in FIG. 3 (C), or the slits may not pass through the center, asshown in FIG. 3 (D).

The slits 21 may be composed of a slit passing through the center andextending to the outer periphery on the end face of the pillar shapedhoneycomb structure 11 and a plurality of slits extending parallel toboth sides of that slit, as shown in FIG. 3 (E), or one slit mayintersect with the other slit at any angle, as shown in FIG. 3 (F), or aplurality of slits may intersect with the other slit at any angle, asshown in FIG. 3 (G).

The slit 21 may entirely interrupted and divided on the end face of thepillar shaped honeycomb structure portion 11, as shown in FIG. 3 (H), orit may be interrupted and divided only near the outer periphery, asshown in FIG. 3 (I), or slits entirely interrupted and divided intersectwith each other, as shown in FIG. 3 (J).

The slits 21 may be formed only near the outer periphery including theouter peripheral wall, on the end face of the pillar shaped honeycombstructure portion 11, as shown in FIG. 3 (K), or they may be providedonly near the outer periphery including the outer peripheral wall, anddivided, as shown in FIG. 3 (L).

(2. Electrically Heating Support)

FIG. 2 is a schematic cross-sectional view of an electrically heatingsupport 30 according to an embodiment of the present invention, which isperpendicular to the extending direction of the cells. The electricallyheating support 30 includes: the honeycomb structure 10; and metalelectrodes 33 a, 33 b electrically connected to the electrode portions13 a, 13 b of the honeycomb structure 10, respectively.

(2-1. Metal Electrode)

Metal electrodes 33 a, 33 b are provided on the electrode portions 13 a,13 b of the honeycomb structure 10. The metal electrode 33 a, 33 b maybe a pair of metal electrode such that one metal electrode 33 a isdisposed so as to face the other metal electrode 33 b across the centralaxis of the pillar shaped honeycomb structure portion 11. As a voltageis applied to the metal electrodes 33 a, 33 b through the electrodeportions 13 a, 13 b, then the electricity is conducted through the metalelectrodes 33 a, 33 b to allow the pillar shaped honeycomb structureportion 11 to generate heat by Joule heat. Therefore, the electricallyheating support 30 can also be suitably used as a heater. The appliedvoltage is preferably from 12 to 900 V, and more preferably from 48 to600 V, although the applied voltage may be changed as needed.

The material of the metal electrodes 33 a, 33 b is not particularlylimited as long as it is a metal, and a single metal, an alloy, or thelike can be employed. In terms of corrosion resistance, electricalresistivity and linear expansion coefficient, for example, the materialis preferably an alloy containing at least one selected from the groupconsisting of Cr, Fe, Co, Ni and Ti, and more preferably stainless steeland Fe—Ni alloys. The shape and size of each of the metal electrodes 33a, 33 b are not particularly limited, and they can be appropriatelydesigned according to the size of the electrically heating support 30,the electrical conduction performance, and the like.

By supporting the catalyst on the electrically heating support 30, theelectrically heating support 30 can be used as a catalyst. For example,a fluid such as an exhaust gas from a motor vehicle can flow through theflow passages of the plurality of cells 18 of the honeycomb structure10. Examples of the catalyst include noble metal catalysts or catalystsother than them. Illustrative examples of the noble metal catalystsinclude a three-way catalyst and an oxidation catalyst obtained bysupporting a noble metal such as platinum (Pt), palladium (Pd) andrhodium (Rh) on surfaces of pores of alumina and containing aco-catalyst such as ceria and zirconia, or a NOx storage reductioncatalyst (LNT catalyst) containing an alkaline earth metal and platinumas storage components for nitrogen oxides (NO.). Illustrative examplesof a catalyst that does not use the noble metal include a NOx selectivereduction catalyst (SCR catalyst) containing a copper-substituted oriron-substituted zeolite, and the like. Further, two or more catalystsselected from the group consisting of those catalysts may be used. Amethod for supporting the catalyst is not particularly limited, and itcan be carried out according to a conventional method for supporting thecatalyst on the honeycomb structure.

(3. Method for Producing Honeycomb Structure)

Next, a method for producing the honeycomb structure according to anembodiment of the present invention will be described.

The method for producing the honeycomb structure according to anembodiment of the present invention includes: a forming step ofobtaining a honeycomb formed body; a drying step of obtaining ahoneycomb dried body; and a firing step of obtaining a honeycomb firedbody.

(Forming Step)

In the forming step, first, a forming raw material containing a ceramicraw material is prepared. The forming raw material is prepared by addingmetal silicon powder (metal silicon), a binder, a surfactant(s), a poreformer, water, and the like to silicon carbide powder (silicon carbide).It is preferable that a mass of metal silicon is from 10 to 40% by massrelative to the total of mass of silicon carbide powder and mass ofmetal silicon. The average particle diameter of the silicon carbideparticles in the silicon carbide powder is preferably from 3 to 50 μm,and more preferably from 3 to 40 μm. The average particle diameter ofthe metal silicon (the metal silicon powder) is preferably from 2 to 35μm. The average particle diameter of each of the silicon carbideparticles and the metal silicon (metal silicon particles) refers to anarithmetic average diameter on volume basis when frequency distributionof the particle size is measured by the laser diffraction method. Thesilicon carbide particles are fine particles of silicon carbide formingthe silicon carbide powder, and the metal silicon particles are fineparticles of metal silicon forming the metal silicon powder. It shouldbe noted that this is the formulation of the forming raw material in thecase where the material of the honeycomb structure is thesilicon-silicon carbide composite material, and when the material ofinterest is silicon carbide, no metal silicon is added.

Examples of the binder include methyl cellulose, hydroxypropylmethylcellulose, hydroxypropoxyl cellulose, hydroxyethyl cellulose,carboxymethyl cellulose, polyvinyl alcohol and the like. Among these, itis preferable to use methyl cellulose in combination withhydroxypropoxyl cellulose. The content of the binder is preferably from2.0 to 10.0 parts by mass when the total mass of the silicon carbidepowder and the metal silicon powder is 100 parts by mass.

The content of water is preferably from 20 to 60 parts by mass when thetotal mass of the silicon carbide powder and the metal silicon powder is100 parts by mass.

The surfactant that can be used includes ethylene glycol, dextrin, fattyacid soaps, polyalcohol and the like. These may be used alone or incombination of two or more. The content of the surfactant is preferablyfrom 0.1 to 2.0 parts by mass when the total mass of the silicon carbidepowder and the metal silicon powder is 100 parts by mass.

The pore former is not particularly limited as long as the pore formeritself forms pores after firing, including, for example, graphite,starch, foamed resins, water absorbing resins, silica gel and the like.The content of the pore former is preferably from 0.5 to 10.0 parts bymass when the total mass of the silicon carbide powder and the metalsilicon powder is 100 parts by mass. An average particle diameter of thepore former is preferably from 10 to 30 μm. The average particlediameter of the pore former refers to an arithmetic average diameter onvolume basis when frequency distribution of the particle size ismeasured by the laser diffraction method. When the pore former is thewater absorbing resin, the average particle diameter of the pore formerrefers to an average particle diameter after water absorption.

The resulting forming raw material is then kneaded to form a green body,and the green body is then extruded to prepare a honeycomb formed body.The honeycomb formed body includes: the outer peripheral wall; and thepartition walls which are disposed on the inner side of the outerperipheral wall and define the plurality of cells each extending fromone end face to the other end face to form the flow passage.

The honeycomb formed body has a part of the partition walls being lostso that some of the cells are connected to each other. By producing sucha honeycomb formed body in which a part of the partition walls is lostso that some of the cells are connected to each other, the connectedcells form the slit(s), and any step of forming the slit(s) by cuttingor the like become unnecessary after the subsequent drying step.Therefore, a production efficiency is improved. It can also eliminate aproblem of wear and damage to processing tools used to form the slit(s),thereby reducing the production cost. Furthermore, when the slit isformed by cutting or the like, it may cause a problem that the slitpenetrates into adjacent cells. However, the present invention forms theslit shape at the stage of forming the honeycomb formed body, so thatthe slit penetration into the adjacent cells can be well controlled.

The honeycomb formed body in which a part of the partition walls is lostso that some of the cells are connected to each other can be producedusing a molding machine having a die in which some of holes are blockedby inserting a pin(s). The shape of the pin is not limited, but forexample, a U-shaped pin 41 as shown in FIG. 4 (A) or a T-shaped pin 42as shown in FIG. 4 (B) may be used.

FIG. 4 (A) shows a top view [1], a side view [2], and a bottom view [3]of the U-shaped pin 41. FIG. 4 (B) shows a top view [1], a side view[2], and a bottom view [3] of the T-shaped pin 42.

It is preferable that a width D1 on an upper surface of each of theU-shaped pin 41 and the T-shaped pin 42 is from 0.9 to 1.2 times adistance of the slit in a width direction (opening distance). Accordingto such a configuration, the generation of a slit portion (also called aburr) that cannot be fully removed by the U-shaped pin 41 and theT-shaped pin 42 can be suppressed. The suppression of the generation ofthe burr leads to easy filling of the slit with a filling material fromthe outer peripheral side. The width D1 on the upper surface of each ofthe U-shaped pin 41 and the T-shaped pin 42 may be from 0.4 to 1.4 mm,for example.

It is preferable that a leg length L1 of each of the U-shaped pin 41 andthe T-shaped pin 42 is substantially the same as a height of a cellblock 44 of a die 43 so that the pin inserted into the die 43 isdifficult to pull out therefrom. The leg length L1 of each of theU-shaped pin 41 and the T-shaped pin 42 may be from 1.5 to 6.0 mm, forexample.

It is preferable that a leg thickness T1 of each of the U-shaped pin 41and the T-shaped pin 42 is preferably 0.9 to 1.1 times the distance ofthe cell block 44 so as to prevent a green body from flowing into theslit forming portion of the die 43 and to prevent the pin from fallingout during molding. The leg thickness T1 of each of the U-shaped pin 41and the T-shaped pin 42 may be from 0.06 to 0.28 mm, for example.

It is preferable that a table length L2 of the U-shaped pin 41 is suchthat the legs of the U-shaped pin 41 are parallelly inserted into theholes of the die 43. For example, the table length L2 of the U-shapedpin 41 may be from 0.45 to 1.3 mm.

It is preferable that a shoulder length L3 of the T-shaped pin 42 issuch that the pin 42 does not penetrate into the partition wall adjacentto the slit. The shoulder length L3 of the T-shaped pin 42 may be from1.1 to 2.6 mm, for example.

FIG. 5 (A) is a schematic plane view for explaining a state where theslit of the honeycomb formed body is formed using the U-shaped pin. FIG.5 (B) is a schematic cross-sectional view of the U-shaped pin and thedie in the state corresponding to FIG. 5 (A). As shown on the left sideof FIG. 5 (A) and FIG. 5 (B), the U-shaped pin 41 can be inserted intoholes of the die 43 of the molding machine and a green body can beextruded from the die in that state to produce a honeycomb formed bodyin which a part of the partition walls 19 is lost to form a linear slit21, as shown on the right side of FIG. 5 (A). By providing a series ofU-shaped pins 41, a longer linear slit can be formed. Also, by providinga plurality of U-shaped pins 41 apart from each other by a predeterminednumber of holes of the die 43, the divided slits can be formed.

FIG. 6 (A) is a schematic plane view for explaining a state where theslit of the honeycomb formed body is formed by using the T-shaped pin.FIG. 6 (B) is a schematic cross-sectional view of the T-shaped pin andthe die in the state corresponding to FIG. 6 (A). As shown on the leftside of FIG. 6 (A) and FIG. 6 (B), the T-shaped pin 42 can be insertedinto the hole of the die 43 of the molding machine and a green body canbe extruded from the die in that state to produce a honeycomb formedbody in which a part of the partition walls 19 is lost to form a linearslit 21, as shown on the right side of FIG. 6 (A). By providing a seriesof T-shaped pins 42, a longer linear slit can be formed. Also, byproviding a plurality of T-shaped pins 42 apart from each other by apredetermined number of holes in the die 43, the divided slits can beformed.

FIG. 7 (A) shows a schematic cross-sectional view of a honeycomb formedbody in which each of cells 18 has a quadrangular cross-sectional shape.FIG. 7 (B) shows a schematic cross-sectional view of a honeycomb formedbody in which each of the cells 18 has a hexagonal cross-sectionalshape. Here, a ratio L/D of a length L to a width D of the slit 21 ispreferably from 1 to 5 when the cell structure is quadrangular, and from1.5 to 8 when the cell structure is hexagonal. It is more preferablethat the ratio L/D is 4 or less when the cell structure is quadrangular,and 6 or less when the cell structure is hexagonal, because thedeformation of the slit 21 can be satisfactorily suppressed. Morepreferably, the ratio L/D is from 1 to 4 when the cell structure isquadrangular, and from 1.5 to 6 when the cell structure is hexagonal.

In FIG. 7 (A) and FIG. 7 (B), a region 45 shown by the dotted line is apartition wall portion located around the slit 21, which is cut at aposition of half the thickness of the partition wall to surround theslit 21. A ratio of an area of the slit 21 to an area of the region 45(opening ratio) is preferably from 67 to 90%. The opening ratio of 90%or less can allow the deformation of the slit 21 to be suppressed moresatisfactorily.

The U-shaped pin 41 or T-shaped pin 42 may be made of any material suchas metals and resins, but it is preferable to use cemented carbide orSUS to prevent deformation or damage during molding.

The honeycomb formed body in which a part of the partition walls is lostso that some of the cells are connected to each other can be produced byextrusion molding of the honeycomb formed body using a molding machinewith a die in which some of the holes are closed. As shown in FIG. 8,the holes of the die 43 can be closed to form a block portion 46,thereby forming the slit at positions corresponding to the cell block 44of the die 43 and the block portion 46 in the honeycomb formed bodyextruded and formed by the molding machine. The block portion 46 may beintegrally formed with the cell block 44 of the die 43, or a separateblock portion 46 made of the same or different material as that of thecell block 44 may be provided between the cell blocks 44 of the die 43.

The honeycomb formed body in which a part of the partition walls is lostso that some of the cells are connected to each other can be produced byextrusion molding of the honeycomb formed body using a molding machinehaving a die and a noodle which is provided on an upstream side of apassage of a forming raw material relative to the die, and in which someof the holes are closed. FIG. 10 illustrates an example of a schematiccross-sectional view of the molding machine 22 to describe a step offorming a kneaded material 23 in the molding machine 22. In the moldingmachine 22, the kneaded material 23 is extruded to pass through a screen24, a noodle 25, and a drawing jig 26, and formed by a die 27 to producea honeycomb formed body 28. The screen 24 is provided to block theinflow of coarse particles of the raw material and to prevent cloggingof the die. The noodle 25 is provided to support the screen 24. Thedrawing jig 26 is provided to draw the kneaded material 23 to a diameterof the die 27. The screen 24, the noodle 25, and the drawing jig 26 areprovided on the upstream side of the passage of the forming raw materialrelative to the die 27, as shown in FIG. 10. In the molding machine 22having such a structure, some of the holes of the noodle 25 can beclosed to form the slit 21 at the positions corresponding to the closedholes of the noodle 25 in the honeycomb formed body 28 extruded from thedie 27.

The honeycomb formed body in which a part of the partition walls is lostso that some of the cells are connected to each other can be produced bypreparing a forming raw material having holes that can form a honeycombformed body in which a part of the partition walls is lost duringextrusion molding by kneading, and then extruding the forming rawmaterial. The forming raw material prepared by the kneading is alsocalled a kneaded material, which is made of soil (ceramic material) andwater, and is generally formed in a cylindrical shape. By forming holesin advance in the part of the kneaded material where the slit is desiredto be formed, it is possible to form a honeycomb formed body in whichthe partition walls are lost at the positions corresponding to the holesduring the extrusion molding.

The honeycomb formed body may have a structure in which a part of thepartition walls is formed thinner than the other partition walls andarranged in the form of slit. By thus producing the honeycomb formedbody in which a part of the partition walls is formed thinner than theother partition walls and arranged in the form of slit, the thinnerpartition walls can be scrapped after the subsequent drying step to formthe slit easily. Thus, by forming a part of the partition walls thinnerthan the other partition walls, rather than completely removing a partof the partition walls, the honeycomb shape can be retained during thedrying and firing steps. A length of the portion where a part of thepartition walls is formed thinner than the other partition walls andarranged in the form of slit is preferably from 50 to 100%, morepreferably from 70 to 100%, of a length of a linear slit of a finalproduct (honeycomb structure), in terms of improving a productionefficiency and production cost. The length of the linear slit of thefinal product (honeycomb structure) may be from 1 to 200 mm.

The honeycomb formed body in which a part of the partition walls isformed thinner than the other partition walls can be produced byextrusion molding of the honeycomb formed body using a molding machinehaving a die in which a part of holes is formed smaller than the otherholes. As shown in FIG. 9, for the holes between the cell blocks 44 ofthe die 43, a hole 47 formed smaller than the other holes can beprovided, whereby a part of the partition walls corresponding to thehole 47 in the honeycomb formed body obtained by the extrusion moldingcan be formed thinner than the other partition walls.

(Drying Step)

The resulting honeycomb formed body is then dried to produce a honeycombdried body. The drying method is not particularly limited. Examplesinclude electromagnetic wave heating methods such as microwaveheating/drying and high-frequency dielectric heating/drying, andexternal heating methods such as hot air drying and superheated steamdrying. Among them, it is preferable to dry a certain amount of moistureby the electromagnetic wave heating method and then dry the remainingmoisture by the external heating method, in terms of being able to drythe entire molded body quickly and evenly without cracking. As forconditions of drying, it is preferable to remove 30 to 99% by mass ofthe water content before drying by the electromagnetic wave heatingmethod, and then reduce the water content to 3% by mass or less by theexternal heating method. The dielectric heating/drying is preferable asthe electromagnetic heating method, and hot air drying is preferable asthe external heating method. The drying temperature may preferably befrom 50 to 120° C.

(Firing Step)

The resulting honeycomb dried body is then fired to produce a honeycombfired body. As the firing conditions, the honeycomb dried body ispreferably heated in an inert atmosphere such as nitrogen or argon at1400 to 1500° C. for 1 to 20 hours. After firing, an oxidation treatmentis preferably carried out at 1200 to 1350° C. for 1 to 10 hours in orderto improve durability. The methods of degreasing and firing are notparticularly limited, and they can be carried out using an electricfurnace, a gas furnace, or the like.

The honeycomb fired body may be used as a honeycomb structure as it is.The method for producing the honeycomb structure with electrode portionsis carried out by, first, applying the electrode portion forming rawmaterial containing ceramic raw materials to the side surface of thehoneycomb dried body and drying it to form a pair of unfired electrodeportions on the outer surface of the outer peripheral wall, across thecentral axis of the honeycomb dried body, so as to extend in the form ofstrip in the flow direction of the cells, thereby providing a honeycombdried body with unfired electrode portions. The honeycomb dried bodywith unfired electrode portions is then fired to provide a honeycombfired honeycomb body having a pair of electrode portions. The honeycombstructure with the electrode portions is thus obtained. In addition, theelectrode portions may be formed after the honeycomb fired body isproduced. Specifically, once the honeycomb fired body is produced, apair of unfired electrode portions may be formed on the honeycomb firedbody, and fired to produce the honeycomb fired body with the pair ofelectrode portions.

The electrode portion forming raw material can be formed byappropriately adding and kneading various additives to raw materialpowder (metal powder, and/or ceramic powder, and the like) formulatedaccording to required characteristics of the electrode portions. Wheneach electrode portion is formed as a laminated structure, the joiningstrength between each metal terminal and each electrode portion tends tobe improved by increasing an average particle diameter of the metalpowder in the paste for the second electrode portion, as compared withan average particle diameter of the metal powder in the paste for thefirst electrode portion. The average particle diameter of the metalpowder refers to an arithmetic average diameter on volume basis whenfrequency distribution of the particle diameter is measured by the laserdiffraction method.

The method for preparing the electrode portion forming raw material andthe method for applying the electrode portion forming raw material tothe honeycomb fired body can be performed according to a known methodfor producing a honeycomb structure. However, in order to achieve lowerelectrical resistivity of the electrode portions than that of thehoneycomb structure portion, it is possible to increase a metal contentratio or to decrease the particle diameter of the metal particles ascompared with the honeycomb structure portion.

Before firing the honeycomb dried body with unfired electrode portions,degreasing may be carried out in order to remove the binder and thelike. As the firing conditions for the honeycomb dried body with unfiredelectrode portions, the honeycomb dried body with unfired electrodeportions is preferably heated in an inert atmosphere such as nitrogenand argon at 1400 to 1500° C. for 1 to 20 hours. After firing, anoxidation treatment is preferably carried out at 1200 to 1350° C. for 1to 10 hours in order to improve durability. The methods of degreasingand firing are not particularly limited, and they can be carried outusing an electric furnace, a gas furnace, or the like.

(4. Method for Producing Electrically Heating Support)

In one embodiment of the method for the electrically heating support 30according to the present invention, a metal electrode is electricallyconnected to each of the pair of electrode portions on the honeycombstructure 10. Examples of the connecting method includes laser welding,thermal spraying, ultrasonic welding, and the like. More particularly, apair of metal electrodes are provided on the surfaces of the electrodeportions across the central axis of the pillar shaped honeycombstructure portion 11. The electrically heating support 30 according toan embodiment of the present invention is thus obtained.

(5. Exhaust Gas Purifying Device)

The electrically heating support according to the above embodiment ofthe present invention can be used for an exhaust gas purifying device.The exhaust gas purifying device includes the electrically heatingsupport and a metallic cylindrical member for holding the electricallyheating support. In the exhaust gas purifying device, the electricallyheating support can be installed in an exhaust gas flow passage forallowing an exhaust gas from an engine to flow.

EXAMPLES

Hereinafter, Examples is illustrated for better understanding of thepresent invention and its advantages, but the present invention is notlimited to these Examples.

Example 1 (1. Production of Green Body)

Silicon carbide (SiC) powder and metal silicon (Si) powder were mixed ina mass ratio of 80:20 to prepare a ceramic raw material. To the ceramicraw material were added hydroxypropylmethyl cellulose as a binder, awater absorbing resin as a pore former, and water to form a forming rawmaterial. The forming raw material was then kneaded by means of a vacuumgreen body kneader to prepare a cylindrical green body (a kneadedmaterial). The content of the binder was 7.0 parts by mass when thetotal of the silicon carbide (SiC) powder and the metal silicon (Si)powder was 100 parts by mass. The content of the pore former was 3.0parts by mass when the total of the silicon carbide (SiC) powder and themetal silicon (Si) powder was 100 parts by mass. The content of waterwas 42 parts by mass when the total of the silicon carbide (SiC) powderand the metal silicon (Si) powder was 100 parts by mass. The averageparticle diameter of the silicon carbide powder was 20 μm, and theaverage particle diameter of the metal silicon powder was 6 μm. Theaverage particle diameter of the pore former was 20 μm. The averageparticle diameter of each of the silicon carbide powder, the metalsilicon powder and the pore former refers to an arithmetic averagediameter on volume basis, when measuring frequency distribution of theparticle size by the laser diffraction method.

(2. Production of Honeycomb Formed Body)

Next, a molding machine having the die structure as shown in FIG. 10 wasprepared. FIG. 11 (A) shows a schematic plane view of the die used inExample 1. The cell block 44 of the die was hexagonal, and the T-shapedpin 42 having the structure shown in FIG. 4 (B) was inserted into thehole between the cell blocks 44 of the die. Table 1 shows the width D1,the leg length L1, the leg thickness T1, and the shoulder length L3 ofthe T-shaped pin 42. The T-shaped pins 42 were provided at intervals oftwo cell blocks from each other in the cells arranged on one straightline.

The resulting cylindrical green body (kneaded material) was formed usingthe above molding machine to produce a honeycomb formed body in which apart of the partition walls was lost so that some cells were connectedto each other. The slits 21 as shown in FIG. 11 (B) were formed on theend faces of the resulting honeycomb formed body, and as a whole, theslits that were interrupted and divided were formed as shown in FIG. 3(H).

(3. Production of Honeycomb Dried Body)

The honeycomb formed body was dried by high frequency dielectricheating, and then dried at 120° C. for 2 hours using a hot air dryer toproduce a honeycomb dried body.

(4. Preparation of Electrode Portion Forming Paste and Production ofHoneycomb Fired Body)

Metal silicon (Si) powder, silicon carbide (SiC) powder, methylcellulose, glycerin, and water were mixed in planetary centrifugal mixerto prepare an electrode portion forming paste. The Si powder and the SiCpowder were blended so that the volume ratio was Si powder:SiCpowder=40:60. Further, when the total of the Si powder and the SiCpowder was 100 parts by mass, methyl cellulose was 0.5 parts by mass,glycerin was 10 parts by mass, and water was 38 parts by mass. Theaverage particle diameter of the metal silicon powder was 6 μm. Theaverage particle diameter of the silicon carbide powder was 35 μm. Theaverage particle diameter of each of those powders refers to anarithmetic average diameter on a volume basis when frequencydistribution of particle diameters is measured by the laser diffractionmethod.

The electrode portion forming paste was then applied to the honeycombdried body with an appropriate area and film thickness by a curvedsurface printing machine and further dried at 120° C. for 30 minutes ina hot air dryer. The honeycomb dried body was then fired in an Aratmosphere at 1400° C. for 3 hours to obtain a honeycomb structure.Table 1 shows a cell pitch of the obtained honeycomb structure and athickness (rib thickness) of the partition wall 19.

The pillar shaped honeycomb structure had circular end faces each havingan outer diameter (diameter) of 100 mm, a height (length in the flowpassage direction of the cells) of 100 mm, and a thickness of the outerperipheral wall of 0.5 mm. The thickness of each partition was 0.19 mm,the porosity of the partition walls was 45%, and the average porediameter of the partition walls was 8.6 μm. The thickness of eachelectrode portion was 0.3 mm. Further, as shown in FIG. 7 (B), the ratioL/D of the length L to the width D of the slit 21, and a ratio of thearea of the slit 21 to the area of the region where the partition wallslocated around the slit 21 were cut at a position of half the thicknessof the slit 21 to surround the slit 21 (opening ratio) were measured.Table 1 shows the measurement results of L/D and the opening ratio.

Example 2

A honeycomb structure in which a part of the partition walls was lost sothat some cells were connected to each other were produced by the samemethod as that of Example 1, with the exception that the U-shaped pin 41having the structure shown in FIG. 4 (A) was inserted into the holesbetween the cell blocks 44 of the die of the molding machine as shown inFIG. 11 (C). The width D1, leg length L1, leg thickness T1, and tablelength L2 of the U-shaped pin 41 are shown in Table 1. The U-shaped pins41 were provided at intervals of two cell blocks from each other in thecells arranged on one straight line. Table 1 also shows the cell pitchof the obtained honeycomb formed body and the thickness of eachpartition wall 19 (rib thickness). On each end face of the obtainedhoneycomb formed body, the slits 21 were formed as shown in FIG. 11 (D),and as a whole, the slits were interrupted and divided as shown in FIG.3 (H).

Example 3

A honeycomb structure in which a part of the partition walls was lost sothat some cells were connected to each other was produced in the samemethod as that of Example 1, with the exception that each cell block 44of the die was quadrangular and the U-shaped pin 41 of the structureshown in FIG. 4 (A) was inserted into the holes between the cell blocks44 of the die as shown in FIG. 12 (A). The width D1, leg length L1, legthickness T1, and table length L2 of the U-shaped pin 41 are shown inTable 1. The U-shaped pins 41 were provided at intervals of three cellblocks from each other in the cells arranged on one straight line. Table1 also shows the cell pitch of the obtained honeycomb formed body andthe thickness of each partition wall 19 (rib thickness). On each endface of the obtained honeycomb formed body, the slits 21 were formed asshown in FIG. 12 (B), and as a whole, the slits were interrupted anddivided as shown in FIG. 3 (H).

Example 4

A honeycomb structure in which a part of the partition walls was lost sothat some cells were connected to each other was produced by the samemethod as that of Example 2, with the exception that the extrusionmolding was carried out to form the slit using a die with some holesclosed without using the U-shaped pin 41. The closed holes in the diewere at the same positions as the holes into which the U-shaped pin 41was inserted in Example 2. The cell pitch of the obtained honeycombformed body and the thickness of each partition wall 19 (rib thickness)are shown in Table 1. On each end face of the obtained honeycomb formedbody, the slit 21 were formed as shown in FIG. 11 (D), and as a whole,the slit was interrupted and divided as shown in FIG. 3 (H).

<Deformation Evaluation>

As shown in FIG. 13, a rate of change of the cell width Db in the slitformed portion relative to the cell width Da in the non-slit formedportion for each of the quadrangular and hexagonal cells:[(Db−Da)/Da]×100(%) was measured, and a degree of deformation of eachhoneycomb structure was evaluated by that rate of change. A lower rateof change means a lower change in the width of the cells in the slitformed portion. The evaluation results are shown in Table 1. As can beseen from Table 1, the rate of change is less than 10% or less than 20%for Examples 1 to 4, indicating that the deformation of the honeycombstructure is well suppressed.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Cell Cell BlockHexagonal Hexagonal Quadrangular Hexagonal Cell Pitch (mm) 1.04 1.041.04 1.04 Rib Thickness (mm) 0.19 0.14 0.13 0.14 Pin (mm) Width D1 0.60.6 1.0 — Leg Length L1 3.0 3.0 3.0 — Leg Thickness T1 0.190 0.125 0.125— Table Length L2 — 1.1 1.0 — Shoulder Length L3 1.75 — — — Slit L/D 3.65.6 3.2 5.6 Opening Ratio (%) 72 81 86 81 Rate of Change less than 10%less than 10% 10% or more less than 10% less than 20%

DESCRIPTION OF REFERENCE NUMERALS

-   10 honeycomb structure-   11 pillar shaped honeycomb structure portion-   12 outer peripheral wall-   13 a, 13 b electrode portion-   18 cell-   19 partition wall-   21 slit-   22 molding machine-   23 kneaded material-   24 screen-   25 noodle-   26 drawing jig-   27 die-   28 honeycomb formed body-   30 electrically heating support-   33 a, 33 b metal electrode-   41 U-shaped pin-   42 T-shaped pin-   43 die-   44 cell block-   45 region-   46 block portion-   47 hole

1. A method for producing a honeycomb structure, the method comprising:a forming step of extruding a forming raw material containing a ceramicraw material to obtain a honeycomb formed body, the honeycomb formedbody comprising: an outer peripheral wall; and partition walls disposedon an inner side of the outer peripheral wall, the partition wallsdefining a plurality of cells, each of the plurality of cells extendingfrom one end face to the other end face to form a flow passage; a dryingstep of drying the honeycomb formed body to obtain a honeycomb driedbody; and a firing step of firing the honeycomb dried body to obtain ahoneycomb fired body, wherein the forming step comprises extruding theforming raw material to produce a honeycomb formed body in which a partof the partition walls is lost so that some of the cells are connectedto each other.
 2. The method for producing the honeycomb structureaccording to claim 1, wherein the forming step comprising using amolding machine having a die in which a part of holes is closed byinserting at least one pin to produce the honeycomb formed body in whicha part of the partition walls is lost.
 3. The method for producing thehoneycomb structure according to claim 1, wherein the forming stepcomprises using a molding machine having a die in which a part of holesis closed to produce the honeycomb formed body in which a part of thepartition walls is lost.
 4. The method for producing the honeycombstructure according to claim 1, wherein the forming step comprises usinga molding machine having: a die; and a noodle provided at the die on anupstream side of a passage of the forming raw material, a part of holesof the noodle being closed, to produce the honeycomb formed body inwhich a part of the partition walls is lost.
 5. The method for producingthe honeycomb structure according to claim 1, wherein the methodcomprises forming a forming raw material having at least one holecapable of forming the honeycomb formed body in which a part of thepartition walls is lost during extrusion molding, and extruding theforming raw material to form the honeycomb formed body in which a partof the partition walls is lost.
 6. The method for producing thehoneycomb structure according to claim 1, wherein the honeycombstructure has at least one linear slit including the cells, the at leastone linear slit being formed by removing a part of the partition walls,in a cross section perpendicular to a flow passage direction of thecells.
 7. A method for producing a honeycomb structure, the methodcomprising: a forming step of extruding a forming raw materialcontaining a ceramic raw material to obtain a honeycomb formed body, thehoneycomb formed body comprising: an outer peripheral wall; andpartition walls disposed on an inner side of the outer peripheral wall,the partition walls defining a plurality of cells, each of the pluralityof cells extending from one end face to the other end face to form aflow passage; a drying step of drying the honeycomb formed body toobtain a honeycomb dried body; and a firing step of firing the honeycombdried body to obtain a honeycomb fired body, wherein the forming stepcomprises extruding the forming raw material to form a honeycomb formedbody in which a part of the partition walls is formed thinner than theother partition walls and arranged in a form of a slit.
 8. The methodfor producing the honeycomb structure according to claim 7, wherein theforming step comprises using a molding machine having a die in which apart of holes is formed smaller than other holes to produce thehoneycomb formed body in which a part of the partition walls is formedthinner than the other partition walls.
 9. The method for producing thehoneycomb structure according to claim 1, wherein the method furthercomprises the steps of: applying an electrode portion forming rawmaterial containing a ceramic raw material to a side surface of thehoneycomb dried body, and drying the applied electrode portion formingraw material to obtain a honeycomb dried body with unfired electrodeportions; and firing the honeycomb dried body with unfired electrodeportions to obtain a honeycomb structure having a pair of electrodeportions, and wherein the pair of the electrode potions are arranged onan outer surface of the outer peripheral wall across a central axis ofthe honeycomb structure so as to extend in a form of strip in the flowpassage direction of the cells.
 10. A method for producing anelectrically heating support, wherein the method comprises a step ofelectrically connecting a metal electrode to each of the pair ofelectrode portions of the honeycomb structure produced by the methodaccording to claim 9.